Ceramic coatings which can improve industrial and aircraft gas turbine performance and durability have been studied for the past forty years. Since gas turbine engines operate at relatively high temperatures, to enhance their energy output, ceramic coatings have been applied to the turbine component base metal to enhance its corrosion resistance and mechanical durability. These coatings also act as thermal barriers to protect the base metallic alloy for prolonged service life. The need exists for a ceramic coating which possesses the desired chemical and physical properties for proper functioning in gas turbines which burn low grade fuels and which subject the turbine component to high temperatures and associated stresses.
In the past, plasma sprayed yttria-stabilized zirconia compositions have been of interest to researchers. Bratton, R. J. and Lau, S. K., "Zirconia Thermal Barrier Coating," Proceedings of First Int'l Conference on the Science and Technology of Zirconia, Cleveland, Ohio, Jun. 16-18, 1980; U.S. Pat. No. 4,055,705. However, these coatings were found to encounter serious coating spalling problems when exposed to combustion environments that contained impurities such as sulfur, sea salt, and vanadium. Although research still continues in the area of these yttria-stabilized zirconia coatings, for example U.S. Pat. No. 4,916,022, there is a need for other candidate materials.
An acceptable ceramic overcoat material must exhibit high chemical stability, along with a high thermal expansion coefficient. The ceramic overcoat material should also have thermal expansion properties similar to the bond coat which is usually MCrAlY or a diffusion aluminide. If the thermal expansion properties are too dissimilar, then the resulting stresses which arise during the cyclic heating cycle could impair performance of the turbine component.
Among the refractory materials, some mixed oxides are known to exhibit high thermal expansion coefficients. "Engineering Properties of Selected Ceramic Materials," Lynch, J. F., et.al., ed., The American Ceramic Society, Inc., 1966. Research has been conducted on calcium silicate, but due to its reaction with sulfur to form CaSO.sub.4, the test results were unsatisfactory. However, refractory titanates have been found to also provide a viable alternative.
A recent study was conducted to determine the superior coating candidate from the group consisting of ZrO.sub.2 *Y.sub.2 O.sub.3, Al.sub.2 O.sub.3 *MgO, ZrO.sub.2 *MgO, Ca.sub.2 SiO.sub.4, and CaTiO.sub.3. Vogan, J. W. and Stetson, A. R., "Advanced Ceramic Coating Development for Industrial/Utility Gas Turbines," DOE Quarterly Reviews, Mar. 4, 1980 and Oct. 13, 1981. The best candidate was determined to be the CaTiO.sub.3 compound for corrosion resistance to Na.sub.2 SO.sub.4 and NaVO.sub.3. However, the CaTiO.sub.3 compound was found to exhibit excessive erosion damage and reacted with magnesium oxide, a fuel additive, as well. Therefore, a new candidate was needed with long term stability characteristics.
In view of the foregoing, there exists a need to provide a new ceramic coating material which exhibits a high thermal expansion coefficient and is nonreactive with corrosive combustion impurities which are present when low grade fuels containing vanadium and sulfur are utilized. It has been discovered that MgTiO.sub.3 and Mg.sub.2 TiO.sub.4 are uniquely the best coating materials available among the refractory titanates for fulfilling this need. Although titanium oxides and magnesium oxides have been employed in the coatings for metal surfaces, the mixed or binary magnesium titanium oxides are novel coatings for protecting the metal surface from corrosive wear. U.S. Pat. No. 4,844,943 discloses the use of magnesium oxides and titanium oxides for protection of metal surfaces from vanadosodic corrosion. However, in such reference the mixed or binary magnesium titanium oxides are not discussed or suggested; only the separate, or single, magnesium and titanium oxides are suggested, and the oxides are formed in situ. U.S. Pat. No. 4,916,022 discloses the use of a titanium oxide, but its use is to enhance the bonding between the MCrAlY and the ceramic overcoating layer and to slow the growth of the alumina scale on the MCrAlY coating layer.
The need for superior ceramic overcoat layers in the industrial gas turbine field is continuing. The prolonging of the useful life of turbine components by protecting the metal substrates from nocuous chemical corrodant attack saves on repair costs, plant down-time, and replacement costs. Furthermore, the coating system imparts thermal insulation to the base metal alloy of the turbine component.